Within the modern technique for cutting or chip removing machining, a variety of different milling tools are found by means of which mainly metallic workpieces can be machined in miscellaneous ways. When milling tools are used, it is natural to designate the machining operations in question under the comprehensive term “milling”. In certain cases, the machining operations have, however, also the character of drilling, viz, when the milling tools are utilized for producing holes in workpieces.
A usually occurring method for genuine milling consists of face milling. In this connection, the milling tool is moved laterally or radially in relation to the rotational axis thereof, the chip removing machining being carried out by means of the peripherical cutting edges of the cutting inserts at the same time as cutting edges along the front end surface of the tool generate the desired, planar surface on the workpiece. The cutting depth in the axial direction is determined by how deep the peripheral edges cut into the material. The chip thickness varies depending on how the cutting inserts enter workpieces. In this respect, the size of the setting angle (κ) of the cutting inserts, which is measured between the machined planar surface and the surface that is generated by the major cutting edges, is crucial in face milling. In modern face milling, the setting angles vary between 90° and 45°. At one and the same cutting depth, the larger angle 90° generates a relatively thick and narrow chip, while the smaller angle 45° generates a thinner and wider chip.
Another milling method consists of plunge-cut milling. In this case, the milling cutter is moved in the axial direction along the surface of the workpiece from which material should be removed, semi-cylindrical or concavely curved grooves being left in the surface. In doing so, the chip removing by the cutting edges is guaranteed along the front end of the tool rather than by peripheral cutting edges.
An additional milling method consists of full-hole helix milling. This method enables the formation of large holes, more precisely by the fact that the tool is entered axially towards the workpiece and is set in motion in a circular, spiral or helix path around the center of the hole to be made, while providing a hole having a greater diameter than the tool itself. Thus, in this case, the tool moves axially as well as radially.
Yet another milling method consists of so-called ramping. The object of such milling is to provide a machined surface which extends other than at a right angle to the rotational axis of the tool. Therefore, in this case, the milling tool is moved simultaneously in the axial as well as the radial direction. Conventionally, it is understood by ramping that the tool is moved in a straight path in the radial direction in relation to the rotational axis while providing a shallow, straight groove in the workpiece, viz, a groove that is delimited by a curved bottom and two straight side surfaces or so-called shoulders. Thus, ramping and full-hole helix milling are closely related with each other in that the single difference between the methods is that the milling in one case generates arched, usually circular arc-shaped shoulders (full-hole helix milling) and in the other case (ramping) straight shoulders are generated. In other words, full-hole helix milling can be said to be a special case of ramping, because the milling tool in both cases is moved axially as well as laterally.
Previously known milling tools for the milling methods described above have most disparate designs depending on the specific field of application. Generally, a dividing line (construction-wise) can be perceived between the milling tools that in the main only undergo to axial feeding motions (such as milling cutters for plunge-cut milling) in contrast to milling tools that primarily undergo radial feeding motions (such as face mills).
In the first-mentioned case (plunge-cut milling), the milling tools can without problem be formed with considerable length in relation to the diameter. Milling cutters for plunge-cut milling may, for instance, have a length/diameter ratio up to 6, i.e., the length of the tool may amount to six times the diameter (6×D). Milling tools that are exposed to large lateral forces can, however, not be made too long. For instance, a face mill having the setting angle 45° of the chip removing cutting edges cannot operate with a greater length/diameter ratio than 2 or 3. At a 90° setting angle, said ration is even smaller. The reason for this difference is that the rigidity of the tool always should be in the direction of chip thickness. Thus, upon milling in multioperation machines, the milling rate is limited by the dynamic stability in the system tool/spindle.
When the cutting depth exceeds a certain value, fed-back vibrations arise, i.e., a so-called regenerative effect. When the tool vibrates, the edges cut a wavy surface in the workpiece and when the same edges later—still vibrating—cut over said wave-shaped surface, a chip is generated having a varying chip thickness. The varying chip thickness results in cutting force variations, which in turn make the system tool/spindle vibrate. The vibration level may become so high that machining in practice becomes impossible to carry out. The regenerative vibrations are reinforced in the direction of chip thickness. For this reason, comparatively long tools (up to 6×D) having small setting angles of the cutting inserts can operate with considerable milling rate, while tools having greater setting angles (45-90°) only can operate with lengths up to 3×D. The above-mentioned regenerative effect is one of the reasons that milling tools that are exposed to large lateral forces cannot be made with a considerable length.
For the users, i.e., different actors in the engineering industry, it is naturally a disadvantage to need different types of tools for each of many different milling methods. This need is disadvantageous not only as a consequence of the fact that different milling cutters in a variety of different dimensions have to be procured and kept in stock, but also as a consequence of the fact that the different milling cutters require different embodiments of cutting inserts which are quickly consumed. The stock-keeping and the administration that is associated therewith, become accordingly extensive.
From U.S. Pat. No. 6,413,023-B1, a milling tool is previously known having cutting inserts, the major cutting edges of which have a setting angle (κ) within the range of 3-35°. However, in that case, the major cutting edges are very short (i.e., less than half the width of the cutting insert) and combined with arched edge portions at the opposite ends thereof. This means that the cutting inserts cannot generate any planar and cylindrical, respectively, surfaces upon face milling and axial milling, respectively, such as plunge-cut milling. Therefore, the milling tool in question cannot be used in a universal way.
Furthermore, from U.S. Pat. No. 4,681,488, a cutting insert is known which is intended for conventional milling tools, and which has a square basic shape and four wiper edges located at an acute angle to a corresponding number of longer major cutting edges. In that case, however, said angle is about 15° or larger. For this reason, there is a risk of emergence of fed-back vibrations in connection with, for instance, face milling or ramping, and therefore the cutting insert is not suitable for a universally usable milling tool.
Objects and Features of the Invention
The present invention aims at obviating the above-mentioned disadvantages of prior art and at providing an improved milling tool. Thus, a primary object of the invention is to provide a cutting tool suited for milling, which is universally usable for many different milling and/or drilling methods and then primarily those described above, i.e., face milling, plunge-cut milling, ramping, and/or full-hole helix milling. Thus, one and the same tool should be usable in order to generate planar as well as cylindrical surfaces having a large smoothness.